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Lyell's "The Student's Elements
of Geology"
Chapter 1: On the Different Classes of Rocks
Introduction:
The Student's Elements of Geology
Chapter 1: On the Different Classes of Rocks
Chapter
2:
Aqueous Rocks
Chapter 3:
Fossils in Strata
Chapter 4:
Consolidation of Strata and Petrifaction
Chapter 5:
Strata Above the Sea
Chapter 6:
Denudation
Geology defined. —
Successive Formation of the Earth’s Crust. — Classification of Rocks according
to their Origin and Age. — Aqueous Rocks. — Their Stratification and imbedded
Fossils. — Volcanic Rocks, with and without Cones and Craters. — Plutonic Rocks,
and their Relation to the Volcanic. — Metamorphic Rocks, and their probable
Origin. — The term Primitive, why erroneously applied to the Crystalline
Formations. — Leading Division of the Work.
Of what materials is the earth
composed, and in what manner are these materials arranged? These are the first
inquiries with which Geology is occupied, a science which derives its name from
the Greek ge, the earth, and logos, a discourse. Previously to
experience we might have imagined that investigations of this kind would relate
exclusively to the mineral kingdom, and to the various rocks, soils, and metals,
which occur upon the surface of the earth, or at various depths beneath it. But,
in pursuing such researches, we soon find ourselves led on to consider the
successive changes which have taken place in the former state of the earth’s
surface and interior, and the causes which have given rise to these changes;
and, what is still more singular and unexpected, we soon become engaged in
researches into the history of the animate creation, or of the various tribes of
animals and plants which have, at different periods of the past, inhabited the
globe.
All are aware that the solid
parts of the earth consist of distinct substances, such as clay, chalk, sand,
limestone, coal, slate, granite, and the like; but previously to observation it
is commonly imagined that all these had remained from the first in the state in
which we now see them—that they were created in their present form, and in their
present position. The geologist soon comes to a different conclusion,
discovering proofs that the external parts of the earth were not all produced in
the beginning of things in the state in which we now behold them, nor in an
instant of time. On the contrary, he can show that they have acquired their
actual configuration and condition gradually, under a great variety of
circumstances, and at successive periods, during each of which distinct races of
living beings have flourished on the land and in the waters, the remains of
these creatures still lying buried in the crust of the earth.
By the “earth’s crust,” is
meant that small portion of the exterior of our planet which is accessible to
human observation. It comprises not merely all of which the structure is laid
open in mountain precipices, or in cliffs overhanging a river or the sea, or
whatever the miner may reveal in artificial excavations; but the whole of that
outer covering of the planet on which we are enabled to reason by observations
made at or near the surface. These reasonings may extend to a depth of several
miles, perhaps ten miles; and even then it may be said, that such a thickness is
no more than 1/400 part of the distance from the surface to the centre. The
remark is just: but although the dimensions of such a crust are, in truth,
insignificant when compared to the entire globe, yet they are vast, and of
magnificent extent in relation to man, and to the organic beings which people
our globe. Referring to this standard of magnitude, the geologist may admire the
ample limits of his domain, and admit, at the same time, that not only the
exterior of the planet, but the entire earth, is but an atom in the midst of the
countless worlds surveyed by the astronomer.
The materials of this crust are
not thrown together confusedly; but distinct mineral masses, called rocks, are
found to occupy definite spaces, and to exhibit a certain order of arrangement.
The term rock is applied indifferently by geologists to all these
substances, whether they be soft or stony, for clay and sand are included in the
term, and some have even brought peat under this denomination. Our old writers
endeavoured to avoid offering such violence to our language, by speaking of the
component materials of the earth as consisting of rocks and soils. But
there is often so insensible a passage from a soft and incoherent state to that
of stone, that geologists of all countries have found it indispensable to have
one technical term to include both, and in this sense we find roche
applied in French, rocca in Italian, and felsart in German. The
beginner, however, must constantly bear in mind that the term rock by no means
implies that a mineral mass is in an indurated or stony condition.
The most natural and convenient
mode of classifying the various rocks which compose the earth’s crust, is to
refer, in the first place, to their origin, and in the second to their relative
age. I shall therefore begin by endeavouring briefly to explain to the student
how all rocks may be divided into four great classes by reference to their
different origin, or, in other words, by reference to the different
circumstances and causes by which they have been produced.
The first two divisions, which
will at once be understood as natural, are the aqueous and volcanic, or the
products of watery and those of igneous action at or near the surface.
Aqueous Rocks.—The
aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a
larger part of the earth’s surface than any others. They consist chiefly of
mechanical deposits (pebbles, sand, and mud), but are partly of chemical and
some of them of organic origin, especially the limestones. These rocks are
stratified, or divided into distinct layers, or strata. The term stratum
means simply a bed, or any thing spread out or strewed over a given
surface; and we infer that these strata have been generally spread out by the
action of water, from what we daily see taking place near the mouths of rivers,
or on the land during temporary inundations. For, whenever a running stream
charged with mud or sand, has its velocity checked, as when it enters a lake or
sea, or overflows a plain, the sediment, previously held in suspension by the
motion of the water, sinks, by its own gravity to the bottom. In this manner
layers of mud and sand are thrown down one upon another.
If we drain a lake which has
been fed by a small stream, we frequently find at the bottom a series of
deposits, disposed with considerable regularity, one above the other; the
uppermost, perhaps, may be a stratum of peat, next below a more dense and solid
variety of the same material; still lower a bed of shell-marl, alternating with
peat or sand, and then other beds of marl, divided by layers of clay. Now, if a
second pit be sunk through the same continuous lacustrine formation at
some distance from the first, nearly the same series of beds is commonly met
with, yet with slight variations; some, for example, of the layers of sand,
clay, or marl, may be wanting, one or more of them having thinned out and given
place to others, or sometimes one of the masses first examined is observed to
increase in thickness to the exclusion of other beds.
The term formation,
which I have used in the above explanation, expresses in geology any assemblage
of rocks which have some character in common, whether of origin, age, or
composition. Thus we speak of stratified and unstratified, fresh-water and
marine, aqueous and volcanic, ancient and modern, metalliferous and
non-metalliferous formations.
In the estuaries of large
rivers, such as the Ganges and the Mississippi, we may observe, at low water,
phenomena analogous to those of the drained lakes above mentioned, but on a
grander scale, and extending over areas several hundred miles in length and
breadth. When the periodical inundations subside, the river hollows out a
channel to the depth of many yards through horizontal beds of clay and sand, the
ends of which are seen exposed in perpendicular cliffs. These beds vary in their
mineral composition, or colour, or in the fineness or coarseness of their
particles, and some of them are occasionally characterised by containing
drift-wood. At the junction of the river and the sea, especially in lagoons
nearly separated by sand-bars from the ocean, deposits are often formed in which
brackish and salt-water shells are included.
In Egypt, where the Nile is
always adding to its delta by filling up part of the Mediterranean with mud, the
newly deposited sediment is stratified, the thin layer thrown down in one
season differing slightly in colour from that of a previous year, and being
separable from it, as has been observed in excavations at Cairo and other
places.1
When beds of sand, clay, and
marl, containing shells and vegetable matter, are found arranged in a similar
manner in the interior of the earth, we ascribe to them a similar origin; and
the more we examine their characters in minute detail, the more exact do we find
the resemblance. Thus, for example, at various heights and depths in the earth,
and often far from seas, lakes, and rivers, we meet with layers of rounded
pebbles composed of flint, limestone, granite, or other rocks, resembling the
shingles of a sea-beach or the gravel in a torrent’s bed. Such layers of pebbles
frequently alternate with others formed of sand or fine sediment, just as we may
see in the channel of a river descending from hills bordering a coast, where the
current sweeps down at one season coarse sand and gravel, while at another, when
the waters are low and less rapid, fine mud and sand alone are carried seaward.2
If a stratified arrangement,
and the rounded form of pebbles, are alone sufficient to lead us to the
conclusion that certain rocks originated under water, this opinion is farther
confirmed by the distinct and independent evidence of fossils, so
abundantly included in the earth’s crust. By a fossil is meant any body,
or the traces of the existence of any body, whether animal or vegetable, which
has been buried in the earth by natural causes. Now the remains of animals,
especially of aquatic species, are found almost everywhere imbedded in
stratified rocks, and sometimes, in the case of limestone, they are in such
abundance as to constitute the entire mass of the rock itself. Shells and corals
are the most frequent, and with them are often associated the bones and teeth of
fishes, fragments of wood, impressions of leaves, and other organic substances.
Fossil shells, of forms such as now abound in the sea, are met with far inland,
both near the surface, and at great depths below it. They occur at all heights
above the level of the ocean, having been observed at elevations of more than
8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above
18,000 feet in the Himalaya.3
These shells belong mostly to
marine testacea, but in some places exclusively to forms characteristic of lakes
and rivers. Hence it is concluded that some ancient strata were deposited at the
bottom of the sea, and others in lakes and estuaries.
We have now pointed out one
great class of rocks, which, however they may vary in mineral composition,
colour, grain, or other characters, external and internal, may nevertheless be
grouped together as having a common origin. They have all been formed under
water, in the same manner as modern accumulations of sand, mud, shingle, banks
of shells, reefs of coral, and the like, and are all characterised by
stratification or fossils, or by both.[
Lyell notes that the vast majority of the earth's rocks, even on the highest
mountains, were formed under water. This, of course would accord well with the
concept of the Biblical flood. Certain aspects of interpretation brought in
later attempt to discredit this possibility. PRS]
Volcanic Rocks.—The
division of rocks which we may next consider are the volcanic, or those which
have been produced at or near the surface whether in ancient or modern times,
not by water, but by the action of fire or subterranean heat. These rocks are
for the most part unstratified, and are devoid of fossils. They are more
partially distributed than aqueous formations, at least in respect to horizontal
extension. Among those parts of Europe where they exhibit characters not to be
mistaken, I may mention not only Sicily and the country round Naples, but
Auvergne, Velay, and Vivarais, now the departments of Puy de Dome, Haute Loire,
and Ardęche, towards the centre and south of France, in which are several
hundred conical hills having the forms of modern volcanoes, with craters more or
less perfect on many of their summits. These cones are composed moreover of
lava, sand, and ashes, similar to those of active volcanoes. Streams of lava may
sometimes be traced from the cones into the adjoining valleys, where they have
choked up the ancient channels of rivers with solid rock, in the same manner as
some modern flows of lava in Iceland have been known to do, the rivers either
flowing beneath or cutting out a narrow passage on one side of the lava.
Although none of these French volcanoes have been in activity within the period
of history or tradition, their forms are often very perfect. Some, however, have
been compared to the mere skeletons of volcanoes, the rains and torrents having
washed their sides, and removed all the loose sand and scorić, leaving only the
harder and more solid materials. By this erosion, and by earthquakes, their
internal structure has occasionally been laid open to view, in fissures and
ravines; and we then behold not only many successive beds and masses of porous
lava, sand, and scorić, but also perpendicular walls, or dikes, as they
are called, of volcanic rock, which have burst through the other materials. Such
dikes are also observed in the structure of Vesuvius, Etna, and other active
volcanoes. They have been formed by the pouring of melted matter, whether from
above or below, into open fissures, and they commonly traverse deposits of
volcanic tuff, a substance produced by the showering down from the air, or
incumbent waters, of sand and cinders, first shot up from the interior of the
earth by the explosions of volcanic gases.
Besides the parts of France
above alluded to, there are other countries, as the north of Spain, the south of
Sicily, the Tuscan territory of Italy, the lower Rhenish provinces, and Hungary,
where spent volcanoes may be seen, still preserving in many cases a conical
form, and having craters and often lava-streams connected with them.
There are also other rocks in
England, Scotland, Ireland, and almost every country in Europe, which we infer
to be of igneous origin, although they do not form hills with cones and craters.
Thus, for example, we feel assured that the rock of Staffa, and that of the
Giant’s Causeway, called basalt, is volcanic, because it agrees in its columnar
structure and mineral composition with streams of lava which we know to have
flowed from the craters of volcanoes. We find also similar basaltic and other
igneous rocks associated with beds of tuff in various parts of the
British Isles, and forming dikes, such as have been spoken of; and some
of the strata through which these dikes cut are occasionally altered at the
point of contact, as if they had been exposed to the intense heat of melted
matter.
The absence of cones and
craters, and long narrow streams of superficial lava, in England and many other
countries, is principally to be attributed to the eruptions having been
submarine, just as a considerable proportion of volcanoes in our own times burst
out beneath the sea. But this question must be enlarged upon more fully in the
chapters on Igneous Rocks, in which it will also be shown, that as different
sedimentary formations, containing each their characteristic fossils, have been
deposited at successive periods, so also volcanic sand and scorić have been
thrown out, and lavas have flowed over the land or bed of the sea, at many
different epochs, or have been injected into fissures; so that the igneous as
well as the aqueous rocks may be classed as a chronological series of monuments,
throwing light on a succession of events in the history of the earth.
Plutonic Rocks (Granite,
etc).—We have now pointed out the existence of two distinct orders of mineral
masses, the aqueous and the volcanic: but if we examine a large portion of a
continent, especially if it contain within it a lofty mountain range, we rarely
fail to discover two other classes of rocks, very distinct from either of those
above alluded to, and which we can neither assimilate to deposits such as are
now accumulated in lakes or seas, nor to those generated by ordinary volcanic
action. The members of both these divisions of rocks agree in being highly
crystalline and destitute of organic remains. The rocks of one division have
been called Plutonic, comprehending all the granites and certain porphyries,
which are nearly allied in some of their characters to volcanic formations. The
members of the other class are stratified and often slaty, and have been called
by some the crystalline schists, in which group are included gneiss,
micaceous-schist (or mica-slate), hornblende-schist, statuary marble, the finer
kinds of roofing slate, and other rocks afterwards to be described.
As it is admitted that nothing
strictly analogous to these crystalline productions can now be seen in the
progress of formation on the earth’s surface, it will naturally be asked, on
what data we can find a place for them in a system of classification founded on
the origin of rocks. I can not, in reply to this question, pretend to give the
student, in a few words, an intelligible account of the long chain of facts and
reasonings from which geologists have been led to infer the nature of the rocks
in question. The result, however, may be briefly stated. All the various kinds
of granites which constitute the Plutonic family are supposed to be of igneous
or aqueo-igneous origin, and to have been formed under great pressure, at a
considerable depth in the earth, or sometimes, perhaps, under a certain weight
of incumbent ocean. Like the lava of volcanoes, they have been melted, and
afterwards cooled and crystallised, but with extreme slowness, and under
conditions very different from those of bodies cooling in the open air. Hence
they differ from the volcanic rocks, not only by their more crystalline texture,
but also by the absence of tuffs and breccias, which are the products of
eruptions at the earth’s surface, or beneath seas of inconsiderable depth. They
differ also by the absence of pores or cellular cavities, to which the expansion
of the entangled gases gives rise in ordinary lava.
Metamorphic, or Stratified
Crystalline Rocks.—The fourth and last great division of rocks are the
crystalline strata and slates, or schists, called gneiss, mica-schist,
clay-slate, chlorite-schist, marble, and the like, the origin of which is more
doubtful than that of the other three classes. They contain no pebbles, or sand,
or scorić, or angular pieces of imbedded stone, and no traces of organic bodies,
and they are often as crystalline as granite, yet are divided into beds,
corresponding in form and arrangement to those of sedimentary formations, and
are therefore said to be stratified. The beds sometimes consist of an
alternation of substances varying in colour, composition, and thickness,
precisely as we see in stratified fossiliferous deposits. According to the
Huttonian theory, which I adopt as the most probable, and which will be
afterwards more fully explained, the materials of these strata were originally
deposited from water in the usual form of sediment, but they were subsequently
so altered by subterranean heat, as to assume a new texture. It is demonstrable,
in some cases at least, that such a complete conversion has actually taken
place, fossiliferous strata having exchanged an earthy for a highly crystalline
texture for a distance of a quarter of a mile from their contact with granite.
In some cases, dark limestones, replete with shells and corals, have been turned
into white statuary marble; and hard clays, containing vegetable or other
remains, into slates called mica-schist or hornblende-schist, every vestige of
the organic bodies having been obliterated.
Although we are in a great
degree ignorant of the precise nature of the influence exerted in these cases,
yet it evidently bears some analogy to that which volcanic heat and gases are
known to produce; and the action may be conveniently called Plutonic, because it
appears to have been developed in those regions where Plutonic rocks are
generated, and under similar circumstances of pressure and depth in the earth.
Intensely heated water or steam permeating stratified masses under great
pressure have no doubt played their part in producing the crystalline texture
and other changes, and it is clear that the transforming influence has often
pervaded entire mountain masses of strata.
In accordance with the
hypothesis above alluded to, I proposed in the first edition of the Principles
of Geology (1833) the term “Metamorphic” for the altered strata, a term derived
from meta, trans, and morphe, forma.
Hence there are four great
classes of rocks considered in reference to their origin—the aqueous, the
volcanic, the Plutonic, and the metamorphic. In the course of this work it will
be shown that portions of each of these four distinct classes have originated at
many successive periods. They have all been produced contemporaneously, and may
even now be in the progress of formation on a large scale. It is not true, as
was formerly supposed, that all granites, together with the crystalline or
metamorphic strata, were first formed, and therefore entitled to be called
“primitive,” and that the aqueous and volcanic rocks were afterwards
superimposed, and should, therefore, rank as secondary in the order of time.
This idea was adopted in the infancy of the science, when all formations,
whether stratified or unstratified, earthy or crystalline, with or without
fossils, were alike regarded as of aqueous origin. At that period it was
naturally argued that the foundation must be older than the superstructure; but
it was afterwards discovered that this opinion was by no means in every instance
a legitimate deduction from facts; for the inferior parts of the earth’s crust
have often been modified, and even entirely changed, by the influence of
volcanic and other subterranean causes, while superimposed formations have not
been in the slightest degree altered. In other words, the destroying and
renovating processes have given birth to new rocks below, while those above,
whether crystalline or fossiliferous, have remained in their ancient condition.
Even in cities, such as Venice and Amsterdam, it cannot be laid down as
universally true that the upper parts of each edifice, whether of brick or
marble, are more modern than the foundations on which they rest, for these often
consist of wooden piles, which may have rotted and been replaced one after the
other, without the least injury to the buildings above; meanwhile, these may
have required scarcely any repair, and may have been constantly inhabited. So it
is with the habitable surface of our globe, in its relation to large masses of
rock immediately below; it may continue the same for ages, while subjacent
materials, at a great depth, are passing from a solid to a fluid state, and then
reconsolidating, so as to acquire a new texture.
As all the crystalline rocks
may, in some respects, be viewed as belonging to one great family, whether they
be stratified or unstratified, metamorphic or Plutonic, it will often be
convenient to speak of them by one common name. It being now ascertained, as
above stated, that they are of very different ages, sometimes newer than the
strata called secondary, the terms primitive and primary which were formerly
used for the whole must be abandoned, as they would imply a manifest
contradiction. It is indispensable, therefore, to find a new name, one which
must not be of chronological import, and must express, on the one hand, some
peculiarity equally attributable to granite and gneiss (to the Plutonic as well
as the alteredunaltered sedimentary strata. I proposed in the Principles
of Geology (first edition, vol. iii) the term “hypogene” for this purpose,
derived from upo, under, and ginomai, to be, or to be born;
a word implying the theory that granite, gneiss, and the other crystalline
formations are alike netherformed rocks, or rocks which have not assumed
their present form and structure at the surface. They occupy the lowest place in
the order of superposition. Even in regions such as the Alps, where some masses
of granite and gneiss can be shown to be of comparatively modern date,
belonging, for example, to the period hereafter to be described as tertiary,
they are still underlying rocks. They never repose on the volcanic or
trappean formations, nor on strata containing organic remains. They are
hypogene, as “being under” all the rest.
From what has now been said,
the reader will understand that each of the four great classes of rocks may be
studied under two distinct points of view; first, they may be studied simply as
mineral masses deriving their origin from particular causes, and having a
certain composition, form, and position in the earth’s crust, or other
characters both positive and negative, such as the presence or absence of
organic remains. In the second place, the rocks of each class may be viewed as a
grand chronological series of monuments, attesting a succession of events in the
former history of the globe and its living inhabitants.
I shall accordingly proceed to
treat of each family of rocks; first, in reference to those characters which are
not chronological, and then in particular relation to the several periods when
they were formed.
Notes
1 See Principles of Geology, by
the Author, Index, “Nile,” “Rivers,” etc.
2 See p. 44, Fig. 7.
3 Col. R. J. Strachey found
oolitic fossils 18,400 feet high in the Himalaya.
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